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72                        MAGNESIA (MAGNESIUM OXIDE)

proportions of extraneous salts (of calcium, alkalies, heavy metals). Crude
magnesia, (calcined magnesite), for technical uses, is in lumps or powder of
colour varying from reddish-white to brownish-grey and may contain
more or less considerable proportions of carbonates, ferric oxide, alumina,
lime, silica and silicates.

Analysis of pure magnesia comprises essentially tests for the commoner
impurities (1-4) ; the analysis of calcined magnesite necessitates various
quantitative determinations, especially of the magnesium oxide, lime,
iron, carbonates and silica (5~7)-

1.  Carbonates,  Insoluble  Substances.-.......i gram,  suspended in  10

c.c. of water and treated with. 10 c.c. of dilute hydrochloric acid (i : i) should
give, in the hot, a clear colourless solution without, evolution of gas.

2.  Chlorides,  Sulphates.--The   nitric  acid   (i : 10)   solution  should
give no turbidity with silver nitrate or barium chloride.

3.  Phosphates (and Arsenic).- -About 2 grams, dissolved in the least
possible quantity of hydrochloric acid and treated with  4.0  c.c. of 10%
ammonium chloride solution and 10 c.c. of ammonia, (1)    : o-qi:o) should
give no turbidity, even after 12 hours.

4.  Heavy Metals, Lime........~i grain, dissolved in dilute hydrochloric

acid, should not be coloured blue by potassium ferroeyanide  (iron) and
should not be rendered turbid by hydrogen sulphide, even when alkaline
with ammonia (copper, kad, iron, zinc] ;   treated with a large excess of
ammonium chloride; and them with ammonia and. ammonium oxa.la.te, it
should not become turbid even after 12 hours (li-uic).

5.  Determination of the Carbonates.   Use is ina.de of out1 of the
methods indicated for the determination of carbon dioxide in limestones
and clays (nee Cement Materials).

6.  Determination of the Silica, Iron, Alumina and Lime, -i 3
grams of the magnesia,, according to its purity, are dissolved in cone-, hydro-
chloric acid (better, in aqua, re-gia) in the hot, the solution being evaporated
to dryness and the residue heated at no", taken up in hydrochloric acid
and filtered.    The insoluble part remaining on the filter, after washing,
igniting and weighing, gives the silica,,

To the filtrate; are added excess of ammonium chloride awl slight excess
of ammonia te> precipitate the iron awl aluminium, which are weighed as
oxides in the usual way. In the filtrate; from this precipitate the lime is
precipitated with ammonium oxalate, which is redissolved in dilute hydro-
chloric acid and again precipitated with ammonium chloride, ammonia and
ammonium oxalate ; in this way the precipitate is freed from magnesia.

7.  Determination of the Magnesia.   -With calcined magnesite the,
total magnesia or that existing as oxide; is required.

(a) TOTAL MAGNESIA (Meyeirheife.r's method). 5 grains of the; finely
powdered sample are; evaporated to dryness with aqua rogia on a steam-
bath, the residue, heated in an oven at 180 -200" for 30 minutes and redis-
solved in a little hot hydrochloric acid, and the solution nltered and the
filtrate made up to i litre. '

Of this solution, go c.c. (*-, i gram of substance1) are treated in a beaker
successively with 5 c.c. of concentrated sulphuric acid, loo c.c. of ainmoniacal free lime and calcu-